This proposal describes several experiments directed towards determining the number and organization of specific gene sequences in the genome of Drosophila melanogaster. The two classes of gene sequences which will be studied are ribosomal RNA and messenger RNA. In order to enrich for the gene sequences coding for these RNA products, a new method of gene isolation is required. The procedure proposed has two novel features: (a) It will allow isolation of DNA sequence(s) coding for any purified RNA transcription product. (b) It will permit the positive identification, in the electron microscope, of the position of the coding sequence on very long single-stranded DNA fragments. This technique will be used to isolate the DNA sequences coding for ribosomal RNA and messenger RNA and to make a detailed study of their structural organization in the genome. The more common technique of reassociation kinetics will be employed to study the number of different messenger RNAs (sequence complexity) present at several stages of development in Drosophila. The measurements obtained will give a minimum estimate of the number of different structural genes in the Drosophila genome. The results of this study and the electron microscopy study on the organization of structural genes will be used to evaluate the current one chromomere:one gene hypothesis. The arrangement of structural genes relative to interspersed middle repetitive DNA sequences will also be determined. The results of the relationship observed will be useful in evaluating some of the relevant features of proposed models for gene regulation.